1. Field of the Invention
The present invention relates to pattern-forming compositions, and more particularly to treatment of pattern-forming thermoplastic compositions that are useful in investment casting that contain undesirably high concentrations of metal, and to investment casting methods employing such compositions.
2. Description of the Prior Art
Various investment casting processes, also known as lost wax processes, have been known for centuries. Through the ages, compositions for the construction of disposable patterns used in such processes have been selected for several characteristics, including such important properties as dimensional reproducibility and the ability to produce a highly accurate surface finish in the molded disposable pattern. Because such properties are critical to many products manufactured by lost wax processes, repeated efforts have been and are being made to improve such properties of pattern-forming compositions.
The quality and properties of an investment casting depend inextricably upon the quality of the disposable pattern, which, in turn, depends upon the characteristics of the pattern-forming compositions of which the disposable patterns are molded. Many thermoplastic pattern compositions have been used or suggested for use in the past. As the name "lost wax" process implies, waxes, such as natural waxes, including beeswax and the like, were originally used as thermoplastic pattern materials. As other pattern materials were sought to improve the properties of disposable patterns, other natural thermoplastic materials, such as gum damar, gum rosin, esparto waxes, and the like, mineral waxes, such as those extracted from soft coal, and the like, and petroleum waxes were adopted for use.
As a result of this search, modified waxes, such as microcrystalline waxes, were developed for use in lost wax processes. More recently, as a result of the continuing efforts of researchers to improve upon and to develop new thermoplastic materials, synthetic thermoplastics have been used as pattern materials or as thermoplastic pattern forming composition modifiers. Those efforts have also resulted in the use by some investment casters of materials other than thermoplastic pattern materials, such as mixtures of metallic salts and mercury.
Disposable thermoplastic patterns are usually formed by heating and melting a thermoplastic composition which is adapted to form a pattern, introducing the molten composition into a mold, and then cooling the composition until it solidifies to form a disposable pattern. As used herein, "melting" of a thermoplastic composition refers to melting the thermoplastic thereof such that the composition becomes fluid even though it may still contain, for example, unmelted solid particulate filler dispersed therethrough. For example, such compositions typically contain solid filler materials. A "filler" is an inert additive in the sense that it does not react chemically with the thermoplastic through which it is dispersed. The filler remains a separate phase and retains its identity throughout the investment casting process. Conventionally, fillers have been solid particulates that are dispersed throughout a continuous phase of the thermoplastic material. Among the filler materials that have been included in minor quantities in thermoplastic pattern-forming compositions may be noted thermoplastic or thermosetting polystyrene powder, especially polystyrene cross-linked with divinylbenzene, and urea powder. U.S. Pat. No. 5,270,360 discloses the use of finely divided poly(methylmetha-crylate) as a filler. Organic acids, such as fumaric acid, adipic acid and isophthalic acid, have also sometimes been used as fillers, usually in amounts of up to 50% by weight of the thermoplastic pattern-forming composition, and in a particle size generally from about 175 to about 250 mesh. Thus, for a typical filler, at least about 90%, preferably 100%, by weight of the particles may pass through a 100-mesh sieve and at least about 50%, preferably about 50%, by weight of the particles pass through a 200-mesh sieve.
Thus, ideal fillers for pattern-forming compositions would provide high thermal conductivity, aid the composition in flowing out of a mold quickly prior to thermal expansion that can cause shell cracking, aid the composition in flowing out of the shell more completely, leave minimal, if any, ash residue in the mold, and result in patterns with smooth surfaces and less shrinkage. Of course, an ideal filler also would be readily available and low cost.
After the disposable thermoplastic pattern is formed, it is removed from the mold, assembled with other patterns, if necessary, and then encased in a mold forming a ceramic material, applied as an aqueous slurry in accordance with one of a variety of known methods, thereby forming a shell or cast about the disposable pattern.
Next, upon hardening of the ceramic, a major portion of the disposable pattern is removed by melting at a moderately elevated temperature by autoclaving, with substantially all of the remainder of the pattern material being removed at a substantially higher temperature by vaporization or burning or both so that, except for any ash residue from the pattern material, the inner surface of the shell or mold is clean. The shell or mold is then ready for a one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled Investment Casting, H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.
By this process, the surface characteristics of the disposable pattern and of the ceramic shell are "transferred" to the final casting. Thus, the above-discussed properties of the pattern-forming composition and any residue therefrom will affect the surface characteristics and metallurgical characteristics of a casting.
Since the pattern material is evacuated by heat and pressure in an autoclave or removed by other methods, such as what is referred to as "flash de-waxing," some residual pattern materials stay behind in the shell, trapped by the configuration of the pattern material, or not liquefied. The remaining wax that is absorbed by the shell and that pattern material that is trapped must be removed at a much higher temperature in what is referred to as a preheat furnace. The preheat ovens in addition to vaporizing the remaining pattern material also preheats the ceramic shall prior to introducing molten metal into this ceramic shell as to avoid the metal from freezing and allowing the molten metal to fill the cavity.
At this step, inorganic impurities contained in the pattern material are reduced to their oxides. The inorganic materials may consist of iron, calcium, and sodium to name a few. When the shell is preheated, if the pattern materials contain these and other inorganic materials, ash from such impurities may be left in the shell.
While the present of ash is, in and of itself, undesirable, the nature of the ash residue is also significant. An ash that is light, puffy and floats away easily with air currents does not create nearly the problem an unctuous or a hard ash that sticks to the inner surface of the shell or mold does. Of particular difficulty is an ash that can result from a high metal content, such as a high iron content, in the thermoplastic composition. When such compositions are heated, the metal tends to form an oxide, which shows up as a glazed ash that is hard and strongly adhered to the inner surface of the shell or mold. The presence of such ash in the shell or mold will, of course, produce a corresponding defect in the surface of the cast part. Therefore, a method is desired for reducing the metal content in the thermoplastic composition, and thereby reducing the amount of such metal oxide ash produced in the shell or mold.